Liquid water content measurement apparatus and method

ABSTRACT

Ice accretion on a probe is detected by determining the change of frequency of a vibrating type ice detector or sensor as ice starts to build up. The rate of change of frequency is determined and is combined with parameters including air velocity and air temperature for providing a signal that indicates liquid water content in the airflow as well as ice accretion on the ice detector.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and method for determiningwith accuracy the liquid water content of ambient air, particularly inrelation to air flows across air vehicles or other structures. Theaccurate and timely measurement of liquid water content (LWC) permitsprompt signalling for activating deicing systems, and also permitssensing atmospheric conditions for reporting or research purposes.

Unheated bodies exposed to airflow laden with supercooled water dropletswill typically accrete ice as the droplets impact the body and freeze.Icing is particularly a problem with air vehicles. Determining when iceis starting to form or predicting when it will form is important inaircraft management of deicing equipment including heaters, which canconsume huge amounts of power. When the air temperature is cold enough,100% of the droplets carried in the airflow will freeze. If thetemperature warms or airflow is increased, the energy balancerelationship is altered. A critical liquid water content is reachedwhere not all of the impinging supercooled water droplets freeze. Thiscritical liquid water content is defined as the Ludlam Limit. The LudlamLimit is described in an article by F. H. Ludlam entitled The HeatEconomy of a Rimed Cylinder. Quart. J. Roy. Met. Soc., Vol. 77, 1951,pp. 663-666. Additional descriptions of the problem are in articles byB. L. Messinger, entitled Equilibrium Temperature of an Unheated IcingSurface as a Function of Air Speed, Journal of the AeronauticalSciences, January 1953, and a further article entitled An Appraisal ofThe Single Rotating Cylinder Method of Liquid Water Content Measurement,by J. R. Stallbrass, Report—Low Temperature Laboratory No. LTR-LT-92,National Research Council, Canada, 1978.

It has been shown that if the LWC increases above the Ludlam Limit, theaccretion characteristics in theory remain unchanged, because excesswater simply blows off or runs off, rather than freezing. Thus, presentsystems for determining liquid water content based on ice accretionsuffer degraded accuracy above the Ludlam Limit. The Ludlam Limit for agiven temperature and airflow is the liquid water content above whichnot all of the water freezes on impact with an accreting surface.

Accretion based ice detectors are frequently designed with probes thatpermit ice build up to a set mass, perhaps taking 30 to 60 secondsdepending on conditions, at which time the presence of ice is enunciatedor indicated, and a probe heater energized to melt the ice. Such icedetectors are well known in the art, and many depend upon a vibratingsensor or probe, with a frequency sensitive circuit set to determinefrequency changes caused by ice accreting on the detector probe.

Liquid water content can be roughly determined by monitoring a signalproportional to the probe icing rate, which again can be determined withexisting circuitry, but accuracy degrades rapidly if the liquid watercontent is above the Ludlam Limit, because a portion of the impingingwater never freezes. In such cases the actual liquid water content willbe under reported, with the Ludlam Limit liquid water content being themaximum that will be reported. Even though the droplet cloud may containadditional liquid water, there will be no indication from such an icedetector that there is additional liquid water in the air flow. Thus,the prior art devices will not discern the actual liquid water contentwhen the Ludlam Limit has been exceeded.

SUMMARY OF THE INVENTION

The present invention relates to determining the liquid water content inan airflow, in particular, air flow past an air data sensing probe on anair vehicle. The amount of the liquid water in the airflow is determinedeven for liquid water content levels above the Ludlam Limit. The presentinvention senses ice growth rate on a vibrating probe type ice detector.The ice growth rate is predictably variable over an accretion cyclebased upon the incremental rate of change of the vibrating probe'sfrequency throughout the sensing cycle. The rate of change of probevibration frequency (df/dt) throughout the ice accretion cycle isdetermined. Further, the rate of frequency change (df/dt)characteristics are demonstrated to be a predictable function of liquidwater, content, even above the Ludlam Limit, meaning that liquid watercontent can be determined at the higher liquid water content level.

The rate of change of probe vibrating frequency is determined for all ora portion of the ice accretion phase of the probe operating cycle,because it has been determined that this rate of frequency change(df/dt) is a function of liquid water content at that time.

In order to measure liquid water content with the present invention, theair speed and the temperature of the ambient air must be known. Thesebasic parameters are readily available from an air data computer, usingoutside instrumentation, such as a pitot tube or a pitot-static tube,and a temperature sensor, such as a total air temperature sensor. Theknown liquid water content at a particular known air speed, temperatureand rate of change of the vibration frequency of a vibrating probe icedetector are determined and combined in a look up table. The values canbe determined by actual icing wind tunnel tests, or test results can beused to derive an algorithm that provides liquid water content when thethree variables, air flow rate (or air speed), temperature and rate ofchange of frequency of vibration caused by ice accretion are known.Although a frequency rate of change is described herein, the rate ofchange of other signals sensitive to ice accretion could be used. Asignal based on the rate of change of ice accretion (but not merely theamount of ice accretion) is a key to proper results.

The overall accretion time has been found to decrease with increasingliquid water content in most cases, but this is not assured. Thisinvention is dependent on ice accretion, and will approach some limit ofusefulness when operating conditions are such that little or no iceaccretes on the probe. This may occur under conditions of warmer airtemperature and high aerodynamic heating, for example.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of the apparatus used fordetermining liquid water content in response to rate of change offrequency caused by commencement of ice accretion on a vibrating probeand for controlling probe heater deicers;

FIG. 2 is a plot of measured rate of change of frequency during iceaccretion at −5° C. temperature, with a constant airspeed of 200 knotswith airflows having three different, but known levels of liquid watercontent in the air flow;

FIG. 3 is a plot similar to FIG. 2 with the indications taken at −10° C.and a constant air speed of 200 knots with the same liquid water contentin the airflows;

FIG. 4 is a plot of rate of change of frequency during ice accretion ofa typical vibrating probe at −5° C. and a speed of 100 knots; and

FIG. 5 is a composite plot of points derived as an average of severalrate of change of frequency values (df/dt) of a test probe as a functionof liquid water content at different air speeds and temperatures.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 illustrates a typical set up for utilization of an existing icedetecting probe and the circuitry for determining liquid water contenteven above the Ludlam Limit. The apparatus 10 includes a vibrating icecollecting or detector probe 12, such as that sold by RosemountAerospace Inc., Burnsville, Minn., as its Model 0871 series. An earlyvibrating, resonant frequency ice detector probe is shown in U.S. Pat.No. 3,341,835 to F. D. Werner et al.

In the present invention, an excitation circuit 14 is used for providingan excitation signal to vibrate the vibrating probe at a resonantfrequency. A known frequency sensing circuit 16 is utilized fordetermining changes of frequency of the vibrating ice detector probe ina conventional manner. The change in frequency is caused by iceaccretion on the surface of the ice detector probe. This design isrecognized to be insensitive to probe contaminants such as dirt andinsects. The rate of accretion of ice is reflected in the rate of changeof frequency. The rate of ice accretion is directly related to theliquid water content of the air. The probe 12 is exposed to airflow asindicated by the arrows 18, and supercooled water droplets will impactand freeze on the probe 12 surface or previously accreted ice at surfacetemperatures below freezing. The signal 34 indicating ice formation canbe used for turning on deicing equipment 36 or other ice protectionsystems for the air vehicle involved and/or notifying the crew of anicing condition. The signal 34 indicating ice formation can be tailoredto the particular air vehicle and its level of tolerance for icebuildup, such that deicing equipment is activated in a timely manner,while nuisance activations are minimized.

The look up tables 26 or algorithm 26A are designed to determine anicing severity level. After a predetermined duration of exposure at aparticular icing condition constituting an icing severity level, or anaggregate of conditions resulting in equivalent ice buildup or impact tothe aircraft, the signal 34 is supplied. The signal may be suppliedcontinually or on a periodic basis until the icing condition abates. Thecalculated df/dt value changes and provides the indication of iceformation, and when correlated to airspeed and temperature is used asthe measured parameter for turning on deicing heaters and determiningliquid water content. The heaters indicated at 20 that are associatedwith the ice detector probe, for removing the ice that has built up onthe probe during the operational cycle, may also be activated with thissignal. The advantage is that reset times may be faster than currentpractice of deicing the probe after a set mass of ice has accreted.

In the present invention, the frequency sensing circuit 16 provides anindication of the change of frequency of the probe 12, and this signalis provided to computer 22 that includes a time input to provide a rateof change of frequency determination section 24. The rate of change offrequency (df/dt) is a function of liquid water content, air temperatureand airspeed and is determined in a matter of milliseconds duringinitial ice accretion, and updated continually until the deicing heatersare turned on. The heaters can be turned on at a selected time after aninitial df/dt signal, or when df/dt reaches a selected value. The probeheaters remain on long enough to deice the probe after which the cyclerepeats. The correlation of the frequency rate change signal to liquidwater content can be provided in a look up table shown at 26, or byentering the parameters into an algorithm in memory section 26A of thecomputer 22. Based upon temperature and airspeed inputs, and themeasured rate of change of frequency over all or a portion of the iceaccretion cycle as shown in FIGS. 2, 3 and 4, the liquid water contentmeasurement can be determined.

The look up tables or algorithm reflecting the measured plots include aninput of the true air speed 28. For example, an input from a pitot tube,or other suitable air speed indicator, that determines the relativevelocity of the airflow 18 past the vibrating probe 12 may be used. Anadditional input parameter is air temperature indicated at 30, which canbe obtained from a known total air temperature sensor, or an ambient airtemperature sensor, as an input to the look up table 26 or algorithmsection 26A.

Air vehicle configuration constants, including for example the aircrafttolerance to ice build up can be an input, as indicated at 27. Thesefactors can insure timely activation, while minimizing nuisanceactivation, of ice protection equipment, and also can insure a morecorrect liquid water content indication.

The known relationship of the liquid water content to the rate of changeof frequency, air speed and air temperature, and if desired, aircraftconfiguration constants, then will provide a signal that is a direct,reliable indication of liquid water content as indicated at 32. Thisliquid water content information can be used for research or analysis ofthe ambient air. Additionally, the output of the look up table andcomputer 22 can be utilized for activating the probe heater 20, as shownby a signal along the line 34, and also can then be used for activatingand turning on the air vehicle surface deicing heaters indicated at 36and/or notifying the crew of an icing condition, which comprise one formof ice protection system.

Utilizing a vibrating type ice detector, and using known air temperatureand airflow velocity, in one plot a temperature of −5° C., and an airvelocity of 200 knots, the results at three different levels of liquidwater content are plotted in FIG. 2. It can be seen that at the knownliquid water content levels of 0.3, 0.75 and 1.2 grams per cubic meter,indicated by the plots 40, 42 and 44, respectively, the rate of changeof resonant vibration frequency of the ice detector probe as iceaccretes on the detector probe provides an indication of the liquidwater content that can be identified quickly. The elapsed time is veryshort before distinct patterns emerge. For example, within 10,000milliseconds a determination of the rate of change in frequency in Hertzper millisecond can be examined and determined from the plotted datapoints. At 20,000 milliseconds the data for each liquid water contentmerge and the plots are clearly defined. From commencement of accretionto about 5,000 milliseconds the data points run together and aresomewhat scattered. The plots or curves are derived using air sampleswith a known liquid water content. All of the liquid water contentsamples used in plotting FIG. 2 have a liquid water content that isabove the Ludlam Limit at the temperature and airflow rates disclosed.

The heaters for deicing the ice detector probe 12 are turned on at theends of the plots in FIGS. 2, 3 and 4. For example, the probe heatersare turned on at the time represented by vertical lines 45 and 46 inFIG. 2 for the plots at 0.75 and 1.2 grams per cubic meter, and areturned on at the time shown by vertical line 48 for 0.3 grams per cubicmeter. The heater turn on signal is given when the ice has built up onthe probe to affect the frequency signal from the probe a desiredamount.

Identifiable results are also achievable with a lower ambient airtemperature, −10° C., as illustrated in FIG. 3, and at the same airvelocity of 200 knots. The plots for 0.3, 0.75 and 1.25 grams per cubicmeter are indicated at 50, 52 and, 54, respectively. The measured datapoints for each liquid water content merge closely together to definedistinct identifiable plots of df/dt in less than 10,000 milliseconds toprovide an indication of the liquid water content, regardless of whetherthe content is above the Ludlam Limit. In FIG. 3, (−10° C. and 200knots) only 0.75 and 1.2 g/m³ plots exceed the Ludlam Limit of liquidwater content.

Again, the probe heaters are turned on where the plots end in FIG. 3,generally along a vertical line 58, for the plots where the liquid watercontent is above the Ludlam Limit, namely plots 52 and 54, and avertical line 56 for the turning on of the deicing heater on thevibrating type deicer probe when the liquid water content is below theLudlam Limit, namely 0.30 g/m³.

FIG. 4 shows further plots of the rate of change of frequency in hertzper millisecond plotted against time, in milliseconds. In this case, thetemperature is −5° C. and airspeed is 100 knots. While somewhat morescattered, the data points can be averaged so that the plots for theliquid water content of 0.30 g/m³, is shown at 60. The 0.30 g/m³ liquidwater content is below the Ludlam Limit while the others are above thelimit. The plot for 0.75 g/m³ is indicated at 62, and the plot for aliquid water content of 1.20 g/m³ is indicated at 64, these plots allshow that the rate of change of frequency, df/dt provides sufficientinformation to indicate the liquid water content within about 15,000milliseconds with reliability. Again, in this instance, the heaters areturned on at a time indicated by vertical lines 66 and 68 for the plotsof 0.75 g/m³ and 1.20 g/m³, respectively, and the heaters are turned onfor the plot for the 0.30 g/m³ at the time line 70.

The rate of change of frequency df/dt, will provide informationindicating the rate of ice accretion in each of the plots, even thoughthe liquid water content may be above the Ludlam Limit. This can providefor early information to the crew of an icing condition and/oractivation of the deicing heaters on the air vehicle to avoid anysubstantial build up of ice. Also, the information on liquid watercontent can be used for research and analysis because the presentinvention gives a reliable indication of liquid water content atsubstantially all ranges of liquid water content.

FIG. 5 is a plot of df/dt averaged data points for different airspeedsto show that there are distinct indications of liquid water content atdifferent air speeds, different liquid water content amounts, anddifferent temperatures such that liquid water content can be determinedreliably.

The points on the plot are derived from an average of approximately 20data point readings near the ends of the plots for corresponding liquidwater content shown in FIGS. 2, 3 and 4, as well as similar data pointstaken at different airspeeds and temperatures as listed in FIG. 5. Forexample, at a temperature of −5° C., three plots are provided for liquidwater contents of 0.3, 0.75 and 1.2 g/m³. Each of these conditions oftemperature and known liquid water content were used to determine df/dtof a vibrating probe at airflows of 100, 150 and 200 knots.

The plot shown at 60 is with 0.30 g/m³ of liquid water at −5° C., and at100, 150 and 200 knots. The change in rate of change of frequency(df/dt) does not show wide swings, but shows definitive changes betweenthe air flows to indicate liquid water content at particular air speedsand temperature based upon the rate of change of frequency.

Plot 62 represents data points for df/dt at −5° C. and 0.75 g/m³ liquidwater content, and shows greater changes between the listed air speeds.

The plot 64 is for −5° C. with a liquid water content of 1.2 g/m³.Again, the rate of change of frequency provides a distinctive signal ateach of the various air speeds to permit direct indication of liquidwater content.

At −10° C., the 0.3 g/m³ liquid water content measuring df/dt results ina plot 66; the 0.75 g/m³ liquid water content results in a plot 68, andthe 1.2 g/m³ liquid water content provides a plot 70. Again, theindividual points shown for the plots 60, 62, 64, 66, 68 and 70 areaverages of df/dt of data points taken shortly before the heater isturned on, or near the right hand end of the plots of data points shownin FIGS. 2, 3 and 4.

In aggregate, the plots of FIG. 5 show that definitive points areestablished at each air speed, temperature, and df/dt condition, so thatupon determining the rate of change of frequency after a selected timefrom the start of ice accretion, the liquid water content at aparticular temperature and a particular air speed can be determined by alookup table or by an algorithm. The look up table values can beextrapolated for different airspeeds and temperatures, so knowing df/dtthe liquid water content can be determined. Also df/dt can give thedesired information on when to turn on the heaters.

The present invention thus uses readily available information forproviding the liquid water content of airflow past a vibrating typeprobe such as an ice detector probe. The determination of the rate ofchange of frequency is a straight forward computation based upon thechange in frequency across a time measurement. The discovery that therate of change of frequency of a vibrating type ice detector probeprovides reliable indications of liquid water content at substantiallyall useful ranges of such liquid water content in ambient air permitsenhanced operation of air vehicles in particular, insofar as deicingequipment is concerned, and enhances the ability to makes liquid watercontent measurements of reasonable quality for research purposes.

The indication of liquid water content is reliably obtained, even whenthe liquid water content is above the Ludlam Limit.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An apparatus for determining the liquid watercontent in air flowing past an aircraft comprising a vibrating icedetector probe mounted on the aircraft and excitable to vibrate at aresonant frequency which changes as ice accretion occurs, a frequencydetermination circuit for determining the frequency of vibration of theice detector probe, and for calculating rate of change of suchfrequency, including a processor, an input to the processor indicatingairspeed, and an input to the processor indicating air temperature, theprocessor correlating parameters comprising the rate of change offrequency, and air velocity and air temperature inputs to previouslyestablished relationships between these parameters characterized by oneof a lookup table and algorithm in the processor to provide an outputindicating liquid water content.
 2. The apparatus of claim 1 includingprobe deicing heaters connected to receive the output for activating theprobe deicing heaters at selected times.
 3. A method of determiningliquid water content in an airflow, for signaling icing conditions foran air vehicle, including providing a vibrating ice detector probe onthe aircraft, determining frequency changes indicating ice accretion onthe ice detector probe, determining the rate of change of frequency ofthe ice detector probe as ice accretes, determining airspeed of the airvehicle, determining air temperature of the airflow and combiningparameters comprising the airspeed, air temperature and the rate ofchange of frequency for correlation to previously establishedrelationships between these parameters characterized by one of a lookuptable and algorithm in the processor for providing an output indicatingliquid water content of the air.
 4. The method of claim 3 including thestep of initiating heaters on the ice detecting probe after a selectedtime.
 5. The method of claim 4 including the step of initiating iceprotection systems on the air vehicle on which the probe is mounted. 6.The method of claim 3 including providing aircraft configurationconstants to the processor.
 7. The method of claim 3, and furthercomprising performing at least one cycle of: heating the probe, andrepeating the steps of determining frequency changes, the temperature,and the airspeed, and performing the operation producing an outputindicating liquid water content of the airflow.
 8. An apparatus fordetermining liquid water content in a body of air, comprising a probesystem comprising a probe, configured to vibrate at a frequency thatchanges as ice accretes on the probe from supercooled water in a body ofair; a logic device, communicatively connected to the probe system,configured to accept inputs from the probe system and provide aparameter indicating a rate of change of frequency of vibration of theprobe, the logic device connected to receive additional input parametersrepresenting a temperature of the body of air and a relative velocity ofthe body of air past the probe system, and the logic device including aset of stored data based on previously established relationships betweenthe parameters as the parameters change to determine liquid watercontent in the body of air; and a heating device, communicativelyconnected to the logic device, and configured for heating the probesufficiently, when activated by an output from the logic device, todiminish ice accreted on the probe.
 9. The apparatus of claim 8, whereinthe logic device includes circuitry to perform at least one cycle oftemporarily activating the heating device to heat the probe, and then tomeasure the rate of change of frequency of the probe after the heatingdevice has been deactivated.
 10. An apparatus for determining liquidwater content in a body of air, comprising a probe, configured tovibrate at a frequency that changes predictably as a function of aquantity of ice accreted on the probe from an amount of supercooledwater in a body of air; a probe sensing circuit, configured to measurethe frequency at which the probe vibrates; a logic device,communicatively connected to the probe sensing circuit and configured toaccept inputs of frequency of vibration, temperature of the body of airand relative airspeed past the probe, the logic device, performingoperations on the inputs, and producing outputs based on the operations;a memory storage device, communicatively connected to the logic device,configured to supply stored data as input to the logic device, includingstored data representing measurements of liquid water content underknown conditions of rate of change of frequency, temperature of the bodyof air and relative airspeed past the probe, and the logic devicecorrelating the rate of change of frequency, the temperature of the bodyof air and the relative airspeed past the probe with the stored data.11. The apparatus of claim 10, wherein there is a heating device on theprobe controlled by the logic device, the logic device configured toperform at least one cycle of temporarily activating the heating deviceto heat the probe, then measuring the rate of change of frequency of theprobe after the heating device has been deactivated, and correlating themeasured rate of change of frequency after heating with the otherinputs.
 12. The apparatus of claim 10, wherein the set of stored datacomprises data on previous tests of the probe system under controlledconditions, configured to serve as a basis for comparison with newinputs.